1. Field of the Invention
The present invention relates to a solid electrolyte gas sensor for detecting a concentration of a specific gas contained in a gas mixture, such as an NOx gas contained in an exhaust gas of an internal combustion engine.
2. Description of Related Art
Sensors for detecting an NOx concentration in an exhaust gas of an internal combustion engine, which are mounted on an exhaust gas passage, are known hitherto. Such sensors are used for controlling an internal combustion engine. An oxygen-ion-conductive solid electrolyte such as a stabilized zirconia is usually used for the sensor. The sensor is composed of a chamber, into which a gas mixture is introduced, and a sensor cell having a pair of electrodes disposed on both surfaces of an oxygen-ion-conductive solid electrolyte layer. One of the sensor cell electrodes which is made of an active material to reduce oxygen of the NOx gas in the mixture into oxygen ions is disposed to face the gas mixture chamber and the other electrode is disposed to face atmospheric air. The concentration of the NOx is measured based on an oxygen ion current flowing through the electrolyte which is proportional to the NOx concentration. However, since oxygen is contained in the gas mixture/such as the exhaust gas, oxygen in the gas mixture is also reduced to oxygen ions together with the oxygen of the NOx gas. Therefore, the gas sensor detects a total amount of oxygen ions of both oxygen in the gas mixture and oxygen in the NOx gas, and, accordingly, it is not possible to measure the ion current proportional only to the NOx concentration.
To eliminate the influence of the oxygen contained in the gas mixture, a gas sensor having an oxygen pumping cell for pumping out the oxygen from the mixture chamber has been proposed, for example, in JP-A-8-29387. The oxygen pumping cell is composed of a pair of electrodes disposed on both surfaces of an oxygen-ion-conductive electrolyte layer. One electrode made of a material which is inactive in reducing oxygen of the NOx gas is disposed to face the mixture chamber, and other electrode is exposed to atmospheric air. By imposing an electric voltage between the pair of electrodes, oxygen in the gas mixture at a vicinity of the pumping cell electrode is ionized and pumped out through the electrolyte. This pumping cell, however, has a drawback that only oxygen at the vicinity of the electrode is ionized and oxygen existing distant from the electrode is difficult to be ionized. Therefore, it is difficult to pump out the oxygen completely from the mixture chamber, and, accordingly, it is difficult to eliminate the influence of the oxygen in the gas mixture in the measurement of the NOx concentration. In other words, it is not possible to tell whether the NOx concentration has actually increased or the amount of oxygen in the gas mixture has increased even if the gas sensor indicates a higher value.
To cope with this problem, a gas sensor having an oxygen sensor cell for checking the oxygen concentration in the mixture chamber has been proposed in a SAE paper No. 960334. The oxygen sensor cell is composed of a pair of electrodes disposed on both surfaces of an oxygen-ion-conductive electrolyte layer. One of the electrodes is exposed to the mixture chamber and the other to atmospheric air. A voltage generated between the pair of electrodes in proportion to the oxygen concentration in the mixture chamber is fed back to the oxygen pumping cell to adjust a voltage to be supplied thereto, so that the oxygen concentration in the mixture chamber is kept at a constant level. This type of the gas sensor, however, has a drawback that the oxygen sensor cell occupying a certain space in the sensor is necessarily required. Accordingly, a space for the oxygen pumping cell is narrowed. Since a pumping capacity of the oxygen pumping cell is proportional to the area of its electrodes, the mount of oxygen pumped out by the pumping cell will decrease as the space for the pumping cell becomes smaller. As the pumping capacity decreases, an amount of the gas mixture introduced into the mixture chamber has to be decreased, resulting in a decrease of the ion current to be detected by the gas sensor. In other words, the sensitivity of the gas sensor is sacrificed. Further, since one electrode of the oxygen sensor has to be exposed to the atmospheric air, a structure of the gas sensor becomes complex.
Though the problems of conventional gas sensors are mentioned with reference to a gas sensor used for detecting the NOx concentration in the gas mixture, the same problems are common to gas sensors detecting other constituent gases such as SOx, H.sub.2 O, CO.sub.2 or the like, as long as a constituent gas concentration is detected by a sensor cell in such a manner that the oxygen ion current resulting from reduction of the constituent gas is measured.